This invention relates to a process of preparing a solid polymer electrolyte type fuel cell employing an ionic conductive solid polymer diaphragm (membrane).
A solid polymer electrolyte type fuel cell has attracted the attention as a source of electricity of an electric automobile and a space craft because it is more compact and may generate higher current density compared with a phosphoric acid type fuel cell. Many proposals as to various electrode structures, methods of preparing catalysts, systems of constituting the fuel cells and the like have been made in this technical field. The electrode structure of a current fuel cell is mainly prepared by loading solid polymer electrolyte around carbon particles supported with a catalyst metal which are eventually present in the solid polymer electrolyte. This conventional electrode for a fuel cell is prepared by contacting the carbon particles previously supported with the catalyst metal with the solid polymer electrolyte.
According to this process of preparation, however, the carbon particles B of which all the surface is covered with the catalyst metal A exist in contact with the solid polymer electrolyte C as shown in FIG. 1, and if a cavity D is formed in the solid polymer electrolyte C, a catalyst metal A' supported on the surface of the carbon particles B facing the cavity D does not contact with the solid polymer electrolyte so as not to function as a catalyst so that the utilization efficiency of the catalyst metal cannot be elevated. Therefore, the voltage drop in the range of the current density practically employed is large and certain limitation to the current density practically employed is present for maintaining a fixed voltage.
In order to elevate the utilization rate of the catalyst metal, a method of directly supporting the catalyst metal on the solid polymer electrolyte membrane (ion exchange membrane) is attempted, but since a metal salt other than that combined with the ion exchange group of the membrane exists in the free state in the membrane, the former is likely to be deposited in the cavity of the membrane to become larger when it is reduced. Accordingly, it is difficult to finely deposit the catalyst metal of high concentration and is nearly impossible to support a fixed amount of the catalyst metal only on the surface of the membrane.